How to Adjust the Air Brakes on Vehicles

by Contributing Writer; Updated June 12, 2017

Vehicles buses and trains mostly rely on air to operate the braking system instead of the hydraulic fluid system that is used in automobiles. The problem with the hydraulic fluid system is that fluid can leak out, resulting in rapid brake system failure. Safety is highly important on large Vehicles because they either have passengers on them or are extremely Vehicles. The modern air brake system was invented by George Westinghouse and utilizes a three-valve operating system. Even though the air brake system is a much safer way to stop large Vehicles regular maintenance and adjustments are required to maintain the proper operation of the braking system. Air brake adjustments can be performed with common tools.

Under The Hood:

 How to Adjust the Air Brakes on a Tractor Trailer

Park the tractor trailer in a safe work area with a level surface.

Apply the air brakes by pulling out on the air brake handle. Walk to the rear of the tractor and place the wheel chocks behind each rear tire of the tractor.

Go back to the front of the truck and make sure the air pressure is at least 120 PSI. If the air pressure is not at 120 PSI, wait for it to build up to 120, then push the air brake handle back in to release the air brakes. Shut the engine off.

Crawl under the rear of the tractor trailer and locate the slack adjusters that are connected to the rear axle housings. The slack adjusters are the components that rotate the brakes up and down.

Look inside the opening of the dust cover on the back of the wheel hub assembly. Locate the drum brakes. The brakes should be about 1/2-inch away from the bottom side of the drum.

Locate the bolt head on the back side of the slack adjuster. This bolt head can be compressed inward to allow the slack adjuster to turn. Position the 1/2-inch drive ratchet with a socket onto the bolt head.

Push the bolt head inward until it stops. Then, turn the bolt head clockwise until the brakes are fully compressed to the bottom side of the drum. Shine the flashlight through the opening on the dust cover to make sure that the brakes are fully compressed to the bottom side of the drums.

Turn the slack adjuster counterclockwise, quarter of a turn, to set the brakes the appropriate distance from the bottom side of the drums. Then, wiggle the bolt head back and forth until the bolt head comes back out and locks in place.

Crank the tractor engine and wait for the air pressure to build back up to 120 PSI. Then, pull out on the air brake handle to set the brakes.

Turn the engine off and remove the wheel chocks. The air brakes on the tractor trailer are now properly adjusted.

Items you will need

  • Wheel chocks

  • Flashlight

  • 1/2-inch drive ratchet

  • 1/2-inch drive socket set

 How to Adjust the Air Brakes on International Trucks

Pull the International truck onto a level surface and apply the air brakes by pulling out on the air brake knob. Leave the engine running.

Get out of the truck and position the wheel chocks in the front and back of the rear wheels.

Release the air brakes by pushing the air brake knob back in.

Slide under the back of the International truck and locate the two slack adjusters on each side of the rear axle. The slack adjusters have an S-cam that extends out of the side of the slack adjuster to the back of the wheel hub.

Locate the adjustment bolt on the back side of the slack adjuster. Position a ratchet and a socket on the adjustment bolt. If the adjustment bolt is spring-loaded, push in on the head of the ratchet to unlock the adjustment bolt so that it can turn. Turn the adjustment bolt clockwise to compress the brake shoes to the inside surface of the brake drums. Once that the adjustment bolt stops turning, the brake shoes should be against the inside of the brake drums. Shine the flashlight through the back side of the wheel hub to ensure that the brake shoes are fully applied to the brake drum.

Turn the adjustment bolt counterclockwise between a 1/4 turn and a 1/2 turn to set the brake shoes the proper distance from the inside surface of the brake drums. Follow this same adjustment process for both slack adjusters.

Items you will need

  • Wheel chocks

  • 1/2-inch drive ratchet

  • 1/2-inch drive socket set

  • Flashlight

 How to Adjust Air Brakes on Heavy Duty Trucks

Start the vehicle and fully charge the air brake air system. Turn off the vehicle.

Place a wheel chock on the front and back side of one wheel to prevent the truck from rolling.

Push in the air brake knob inside the vehicle to release the vehicle brakes.

Locate the slack adjuster. It is on the inside of the brake drum, next to the axle and wheel assembly, under the truck.

Locate the adjusting bolt on the rear of the slack adjuster. This bolt will need to be turned to adjust the brakes.

Turn the bolt clockwise with a box-end wrench and watch the slack adjuster. If the slack adjuster moves inward or toward the front of the truck, the brake shoes are tightening up to the brake drum, which is what you want. If the slack adjuster moves outward or toward the rear of the truck, turn the adjusting bolt counterclockwise to tighten the brake shoe to the brake drum. Continue turning the adjusting bolt until the brake shoes are fully seated against the brake drum.

Tap the edge of the brake drum with a hammer. The sound should be a "thud" when the brake shoes are seated fully against the brake drum.

Turn the adjusting bolt one quarter-turn counterclockwise or the opposite direction that was needed to tighten the brake shoes with a box-end wrench. Tap the edge of the brake drum with a hammer. If it is a "ringing" sound, the brakes are adjusted. If the sound is a "thud," continue turning the adjusting bolt another quarter-turn, then recheck the brake drum sound by tapping on the drum with a hammer. Repeat this process until a ringing sound comes from the brake drum. The brake is adjusted.

Repeat this process to all of the slack adjusters that are mounted on the vehicle.

Pull out on the air brake knob inside the vehicle to set the vehicle brakes.

Remove the wheel chocks.

Items you will need

  • Two wheel chocks or wooden blocks

  • Box-end wrench

  • Hammer

  • Measuring tape

  • Flat-head screwdriver